The present invention relates to electric transformers, especially distribution transformers, and to the protective equipment therefor. More particularly, the invention relates to apparatus for protecting distribution transformers from damage due to lightning induced surge currents entering the secondary windings of the transformer from the low voltage side. Still more particularly, the invention relates to a low-voltage, ruggedly constructed surge arrester of the metal oxide variety having a high energy handling capability.
Distribution transformers convert primary, high voltage levels, such as 2.4 to 34.5 KV, to secondary, low voltage levels, low voltage typically being defined as 1200 volts and less. Most typically, secondary side voltage levels of distribution transformers are 120/240 volts or 240/480 volts. Distribution transformers include primary and secondary windings which are enclosed in a protective metallic housing. A dual secondary voltage, such as 120/240 volts, is achieved by constructing the transformer secondary winding in two halves or sections. One end of each of the two winding sections is electrically joined at a predetermined point and typically grounded at this point of interconnection. In this configuration, when the transformer is energized, the voltage between the grounded interconnection point and each line potential terminal will be the same, i.e., 120 volts, and will be equal to one half the voltage between the two ungrounded ends, i.e., 240 volts.
The primary or high voltage terminals of distribution transformers are conventionally designated as the H.sub.1 and H.sub.2 bushings. The low voltage or secondary side line-potential terminals are designated as X.sub.1 and X.sub.3, while the low voltage grounded neutral bushing or terminal is designated as X.sub.2.
The majority of distribution transformers are designed for pole mounting; however, some are built for pad or platform mounting. Regardless of mounting type, distribution transformers are susceptible to damage from lightning induced surges entering their windings. When a lightning surge occurs, the voltage appearing across the primary winding may exceed the insulation strength of the winding, resulting in a flash-over across or through the winding insulation, thereby causing the transformer to fail. It has been conventional practice to provide overvoltage protection for distribution transformers by means of surge arresters applied to the primary, high voltage winding. More specifically, in the case of single phase distribution transformers in which both primary bushings H.sub.1 and H.sub.2 are at line potential, surge arresters have typically been connected between H.sub.1 and ground and between H.sub.2 and ground. In applications in which primary bushing H.sub.1 is at line potential and H.sub.2 is grounded, it is common to connect a single surge arrester between H.sub.1 and grounded H.sub.2. The surge arrester's function is to provide a path by which lightning induced current is diverted to ground, thus preventing flashover of the transformer's winding insulation.
Investigations have been made in recent years concerning lightning induced failures of common designs of overhead and pad mounted distribution transformers. These investigations revealed that despite the presence of state-of-the-art primary-side lightning protection as described above, many such transformer failures are attributable to lightning induced surges entering the transformer via the normally unprotected low voltage terminals, causing failure of the high voltage winding due to the induced voltages. While lightning induced currents entering the low voltage bushings are normally non-destructive, current surges over 5,000 amps are not uncommon. Secondary surges in the order of 3,000 amps can result in potentially destructive induced voltages in the primary winding which may cause the transformer to fail. Thus, it has been determined that primary side arrester protection of the high voltage winding is ineffective in preventing transformer damage due to lightning induced surge currents injected in the secondary windings.
Lightning induced surge currents can enter the low-voltage or secondary terminals of a distribution transformer in three basic ways. The first and most obvious way is due to direct lightning strikes on secondary service conductors In this case, surge currents are forced through the transformer secondary windings on their way to ground at the transformer neutral X.sub.2. This mode of current surge may involve only one half, or the entire secondary winding.
A second possible mode of surge current injection into the low voltage windings of a distribution transformer is due to lightning discharge into the ground near a secondary service point. Such a discharge can cause a local elevation of ground potential resulting in ground currents flowing outward from the discharge point back toward the transformer's grounded neutral X.sub.2. Some of this current can flow through the transformer secondary windings via the grounded transformer neutral resulting in a low-side current surge.
The third way that surge currents enter low-voltage windings may be less obvious than the others, but is perhaps the most common in occurrence. Lightning strikes to overhead primary-side phase conductors are conducted to ground at the service pole supporting the transformer by a ground wire running down the pole. The surge arrester connected to the primary winding of the distribution transformer forms one path for the surge from the phase conductor to flow to this ground connection. Where there is an overhead neutral conductor, it is connected directly to the pole ground. Since the transformer neutral X.sub.2 is also connected to this ground wire, part of the current discharged by the primary side surge arrester can be diverted into the secondary windings of the transformer via the secondary side.
In each of the last two cases, surge current may enter the grounded neutral terminal X.sub.2 of the low-voltage winding and divide through the two halves of the winding, exiting by way of secondary line terminals X.sub.1 or X.sub.3, or both. For such current to flow through the transformer, there must be a path through the customer load or customer meter gaps, or across gaps in the customer's wiring. Where such a path exists, the amount of surge current conducted through the transformer secondary windings will be dependent both on the amount of customer load connected at the time of the surge and, more significantly, on the ratio of the resistance of the pole ground to the resistance of the customer ground. If the pole ground has a resistance less than that of the customer ground, the current level within the transformer should be well below that required to produce an insulation failure within the windings.
Three-wire surge injection occurs where surge current enters the transformer through X.sub.2 and departs from the transformer through both X.sub.1 and X.sub.3. Two-wire surge injection occurs in two situations. First, it may occur when the surge enters the transformer through X.sub.2 and departs from the transformer through either X.sub.1 or X.sub.3. This can occur when only one customer meter gap fires, or when the load on the service conductor connected to X.sub.1 is substantially different from that on X.sub.3. Two-wire injection may also occur when a surge enters either X.sub.1 or X.sub.3 and exits to ground through X.sub.2.
Depending upon their design, distribution transformers tend to be particularly affected by certain types of surges. More specifically, transformers having uncompensated winding constructions, i.e., non-interlaced low voltage windings, are particularly affected by both three-wire and two-wire surge injection. Transformers having compensated winding constructions, i.e., interlaced low voltage windings, are only affected by two-wire surge injection. The majority of modern day distribution transformers have non-interlaced low voltage windings, and thus are particularly susceptible to damage from both three-wire and two-wire surge injection.
In an effort to protect distribution transformers from such secondary-side surges, various schemes have been employed. First, constructing the transformers with interlaced secondary windings provides good protection from three-wire surges, but, as explained above, two of the most common types of secondary surges result in two-wire surge injection and interlaced windings offer no protection from such surges. Further, transformers having interlaced windings also are more expensive than those with non-interlaced windings.
Alternatively, or additionally, extra primary winding insulation may be added to provide some protection from both two and three-wire surge injection. This technique is relatively expensive, however, and does not prevent surges from entering the transformer, but merely serves to raise the damage threshold level of the transformer.
Recently, surge arresters of the metal oxide varistor (MOV) type have been applied between secondary-side phase terminals, X.sub.1 and X.sub.3, and the grounded neutral terminal, X.sub.2. MOV disks are variable resistors which provide either a high or a low impedance current path through the disk's body depending on the voltage that appears across the MOV disk. More specifically, at the power system's steady state or normal operating voltage, the MOV disk has a relatively high impedance. As the applied voltage is increased, gradually or abruptly, the impedance of the MOV disk progressively decreases until the voltage appearing across the disk reaches the disk's "breakdown" or "turn-on" voltage, at which point the disk's impedance dramatically decreases and the disk becomes highly conductive. Accordingly, if the arrester is subjected to an abnormally high transient over-voltage, such as may result from a lightning strike or power frequency overvoltage, the MOV disk becomes highly conductive and serves to conduct the resulting transient current to ground. As the transient over-voltage and resultant current dissipate, the MOV disk's impedance once again increases, restoring the arrester and the electrical system to their normal, steady state condition.
MOV type secondary surge arresters have been shown to provide adequate two and three wire surge protection for low energy surges of, for example, 10,000 amps or less. Some manufacturers of such arresters claim their arresters are capable of safely dissipating surges of 20,000 amps. However, to date, such MOV secondary arresters have not had the even higher energy discharge capability desirable. Further, state-of-the-art MOV secondary surge arresters are expensive to manufacture due to the precise machining and collaring that is currently required on the MOV disks.
Accordingly, there remains a need in the industry for a low voltage surge arrester capable of protecting a distribution transformer from damage or destruction caused by surge currents that are injected into the secondary windings. Preferably, such an arrester would be effective against short duration surges of 40,000 amps, would be weatherproof and durable, and be of a rugged low cost construction. Such an arrester that could be employed in under oil applications would also be desirable.
Other objects and advantages of the present invention will become apparent from the following description.